Traditional techniques for introducing a printed image onto a recording material include letterpress printing, gravure printing, and offset lithography. All of these techniques require the preparation and use of a lithographic printing plate having an ink-receptive outer surface and a hydrophilic support or substrate. As used herein, the term “lithographic” is meant to include various terms used synonymously such as offset, offset lithographic, planographic, and others known in the art, and is meant to refer to type of lithographic printing plate where printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When the suitably prepared surface is contacted with fountain solution (or water) and a lithographic printing ink, the background or non-image areas retain the water and repel the ink while the image areas repel water and retain ink.
Aluminum has been used for many years as a substrate for lithographic printing plates and their precursors (before imaging). The aluminum substrate is usually treated in a number of ways to improve adhesion of the image areas and to provide desired hydrophilicity to the background of the image.
Some lithographic printing plate precursors are prepared by coating appropriate radiation-sensitive coatings onto the substrate, imaging with appropriate radiation, and developing with an alkaline processing solution to remove non-image areas.
Recent developments in the field of printing plate precursors concern the use of radiation-sensitive compositions that can be imaged by means of lasers or laser diodes, and more particularly, that can be imaged or developed on-press. Laser exposure does not require conventional silver halide graphic arts films as intermediate information carriers (or “masks”) since the lasers can be controlled directly by computers. High-performance lasers or laser-diodes that are used in commercially-available image-setters generally emit radiation having a wavelength of at least 700 nm, and thus the radiation-sensitive compositions are required to be sensitive in the near-infrared or infrared region of the electromagnetic spectrum. However, other useful radiation-sensitive compositions are designed for imaging with ultraviolet or visible radiation.
There are two possible ways of using radiation-sensitive compositions for the preparation of printing plates. For negative-working printing plates, exposed regions in the radiation-sensitive compositions are hardened and unexposed regions are washed off during development. For positive-working printing plates, the exposed regions are dissolved in a developer and the unexposed regions become an image.
Other methods of preparing lithographic printing plates involve laser ablation systems in which organic materials in an imaging layer on the substrate are removed by high laser power. “Ablation” refers to the physical disruption and degradation of the organic materials, producing gases and organic debris.
A variety of laser ablation lithographic printing plate precursors are described in numerous literature publications including but not limited to, EP 580,393 (Lewis et al.) that describes a precursor having an IR-absorbing layer on a substrate, which layer is coated with a surface silicone layer that repels ink. Other embodiments include a hydrophilic poly(vinyl alcohol) surface layer. Still other precursors in this publication have only 1 or up to 3 layers on my substrate.
Still other laser-ablatable lithographic printing plate precursor constructions are described in EP 678,380 (Lewis et al.) and EP 1,088,653 (Rorke et al.), and U.S. Pat. No. 5,493,971 (Lewis et al.), U.S. Pat. No. 6,186,967 (Rorke et al.), U.S. Pat. No. 6,192,798 (Rorke et al.), U.S. Pat. No. 6,352,028 (Ellis), U.S. Pat. No. 6,357,352 (Rorke et al.), U.S. Pat. No. 6,489,975 (Rorke et al.), and U.S. Pat. No. 6,497,178 (Rorke et al.), and U.S. Published Patent Application 2002/0124755 (Lewis).
U.S. Pat. No. 5,605,780 (Burberry et al.) describes a laser-ablatable lithographic printing plate precursor that includes an image-forming layer comprising an infrared radiation absorbing agent dispersed within a film-forming poly(cyanoacrylate) binder.
In general, ablative imaging requires relatively high exposure energies, for example about 1000 mJ/cm2, which means that customers need a unique high power imagesetter for imaging such precursors. Thus, the customers will need individual imagesetters for ablation and other imagesetters for the lower energy imaging techniques (non-ablation). This creates a need for additional equipment and energy costs, and training needs, for the customer. It would be desirable to use the same equipment for both laser-ablative and non-laser-ablative imaging.